Crush saffron with a pinch of sugar using a mortar or the back of a spoon.
Add 3–4 tablespoons hot (not boiling) water. Let it steep for at least 10 minutes.
Pat chicken dry and season with 1/2 teaspoon salt and black pepper.
Heat 2 tablespoons oil in a wide pot over medium-high. Sear chicken until golden on both sides, 6–8 minutes total. Transfer to a plate.
Add 1 tablespoon oil and the onions. Cook until lightly golden, 8–10 minutes. Stir in garlic, turmeric, and cinnamon for 30 seconds.
Add tomato paste; cook 1–2 minutes to deepen color and flavor.
Return chicken to the pot. Add broth, 1/2 teaspoon salt, and half of the bloomed saffron.
Bring to a simmer, cover, and cook over low heat until chicken turns tender, 30–40 minutes. Uncover and reduce the sauce to a spoon-coating consistency. Finish with lemon juice. Taste and adjust salt.
Rinse basmati until water runs mostly clear. Soak for 20–30 minutes in cool water, then drain.
Bring a large pot of water to a rolling boil. Stir in 2 tablespoons salt.
Add rice and cook 5–7 minutes until the grains lengthen and feel al dente (soft outside, firm center). Do not overcook.
Drain in a colander and rinse briefly with warm water to remove excess starch.
Rinse barberries and drain well.
Melt butter in a small pan over low heat. Add barberries and sugar. Stir gently for 1–2 minutes until glossy. Do not brown—they turn bitter.
Add 1–2 teaspoons bloomed saffron and set aside.
Place the rice pot back on the stove. Add 2 tablespoons oil. For yogurt tahdig, mix 1 cup parboiled rice with yogurt and 1 teaspoon saffron water; spread into an even layer. For potato tahdig, layer potato slices over the oil. For plain tahdig, just heat the oil until shimmering.
Pile the remaining rice into a mound. Drizzle with most of the remaining saffron water.
Wrap the pot lid with a clean towel to catch steam. Cover and cook over medium heat for 8 minutes to set the crust.
Reduce heat to low and steam for 30–35 minutes until the rice turns fluffy and the tahdig crisps.
Reserve a cup of white rice and mix it with a spoonful of saffron water for a vibrant topping.
Fluff the rice. Gently fold in half the barberries.
Plate the saffron rice, top with remaining barberries and saffron rice, and garnish with pistachios or almonds.
Serve chicken alongside or spoon the sauce around the rice. Keep tahdig intact when unmolding.
